Music can be enjoyed live in front of the stage in a theatre. However, it is more common that one enjoys music through radios, televisions, DVD home theatres, MP3 players, multimedia personal computers, etc. In these entertainment devices, transducers such as loudspeakers, which convert electric signals into physical sound waves, are used to reproduce the music. However, the quality of music reproduction is typically poor in the low frequency portion so that there are continual demands for bass improvement. The quality problem is due to the physical limitations of the electro-acoustic transducer in dimension and structure. For instance, the pipe organ (Werkprinzip) requires use of an open pipe of length 32 feet (around 10 meters) to produce the CO tone in 16.35 Hz as reported in Eargel, J. M., Music, Sound, and Technology, Second edition, Van Nostrand Reinhold, 1995, the disclosure of which is incorporated by reference herein. Therefore, it is difficult to satisfy the requirements for good low-frequency reproduction in small churches or in general applications.
There have been some techniques in the art to improve the low frequency response of music reproduction devices. In one example, the difficulty of smaller-size pianos with shorter strings to reproduce the CO tone and other low frequency tones can be overcome by using strings that are thicker and stiffer, and that are stretched less tightly. However, it results in a certain degree of inharmonic distortion. In another example, a moving-coil loudspeaker can use a stronger magnetic field and a bass-reflex tube to extend its low frequency range to around ⅓ octave from its low cut-off frequency. This method has been employed in commercial products, e.g., in a product produced by TOA Corporation with model number SW-46S-UL2, which can provide a 30 Hz low frequency response using an 18 inch woofer and base-reflex design.
Owing to the cost and space of the bass implementation for pipe organs of small churches, a technique called “acoustic bass” was used and known as early as the 1700's. The organ manufacturers made two pipes sounding together to get lower notes. For instance, C4 and G4 pipes are used together to get the C3 note. A similar method can be applied to pianos with shortened strings.
Note that in the above-mentioned approach, the sound of the intended frequency is not present, but human listeners can still perceive the presence of this frequency. This phenomenon is known as the residue pitch effect, referred to also as the phenomenon of the missing fundamental. Moor, B. C. J., An Introduction to the Psychology of Hearing, Chapter 5, Fourth edition, Academic Press, 1997, provides background information of this phenomenon, the disclosure of which is incorporated by reference herein. Basically, the residue pitch effect is a psycho-acoustic effect in that the residue pitch (harmonics) of a tone can be perceived by human listeners as the presence of the fundamental frequency even if the fundamental frequency is missing or masked by other noise. The residue pitch effect has been used in U.S. Pat. No. 5,930,373 and U.S. Pat. No. 6,285,767 to enhance bass, resulting in the extension of the low cut-off frequency of a speaker by 1 to 1.5 octaves.
In U.S. Pat. No. 5,930,373, a method for introducing residue harmonics of low-frequency signal components into a sound signal is disclosed. In this method, the sound signal is partitioned into a high frequency signal and a low frequency signal. The low frequency signal is further partitioned into a number of signal components in different frequency bands. Residue harmonics are generated for each of these signal components. The residue harmonics are weighted and added to the original sound signal. In the generation of residue harmonics, this reference suggests that nonlinear transformation may be used.
In U.S. Pat. No. 6,285,767, a sound enhancement system that enhances the perception of low-frequency signal components in a sound signal is disclosed. In this patent, the fact that low-frequency signal components can give rise to harmonics generated by the nonlinearity of human ears is recognized. To emphasize the presence of these harmonics such that the original low-frequency signal components are more easily perceived, the disclosed sound enhancement system de-emphasizes the mid-frequency components originally in the sound by purposefully reducing their power levels.
In U.S. Pat. No. 6,410,838, a musical signal synthesizer for synthesizing complex musical sound waveforms rich in harmonics is disclosed. The waveforms are generated by means of a feedback loop and a simple nonlinearity is used to introduce the harmonics into the signal.
However, there remains a need in the art for improved methods for bass enhancement by incorporating human physiology into consideration. In one aspect, there is a need for improved methods that are based on the nonlinear response of the human ear to enhance the perception of bass frequencies. Music with bass enhancement by such improved methods has the advantage that it sounds more natural to human beings. In contrast, music enhanced by non-ear-based methods appears more artificial in perception. In another aspect, there is a need for improved methods that allow the removal of the low-frequency signal components in a sound but human listeners can still psychologically perceive the presence of such low-frequency signal components. The absence of low-frequency signal components implies that the bass quality achieved by a sound generator is nothing to do with the quality of the reproduced sound or music. Therefore, a cheaper sound generator can be used instead of a more-expensive, bass-enhanced sound generator, thereby lowering the material cost. Moreover, the removal of such signal components avoids human ears to duplicate generation of same or similar residue harmonics so that it can prolong the heavy-bass music listening time for human listeners with less demand on ears. Finally, removing the low-frequency portion is also a means for combating against unauthorized copying of the original sound signal via tapping an analog output of a device, while not degrading any sound quality in the bass content listening. The problem of unauthorized copying of a sound signal by tapping at the analog output of the device is known as the analog loophole problem.